mmc: sdhci-s3c: Add device tree support

Add device tree based discovery support for Samsung's sdhci controller.

Cc: Ben Dooks <ben-linux@fluff.org>
Cc: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Thomas Abraham <thomas.abraham@linaro.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>